Monoclonal antibodies (MAbs) have been utilized with immunohistochemical methods for the (a) detection of occult carcinoma in surgical and cytology preparations, (b) phenotyping of malignant cell populations, (c) differentiation of histologic tumor types, and (d) identification of various cellular products. MAbs with selective reactivity against tumor associated antigens have specifically been adapted for use with cytologic preparations including cytospins, membranes, and fine needle aspiration biopsies for the detection and differentiation of carcinomas from benign cell types and other cancerous lesions.